Pure zirconium dioxide is characterized by a volume change associated with monoclinic-tetragonal crystal phase transformation at a temperature between about 1000.degree.-1200.degree. C. Such volume change limits the utility of zirconia in refractory applications.
Commercial use of zirconia as a refractory substrate has resulted from the development of stabilized cubic zirconia with additives such as calcium, magnesium, and yttrium oxides. Extensive research has been directed to the study of the functional relationship between the monoclinic/tetragonal/cubic crystal phases of zirconia and the stabilizer additives and the consequential effect on the physicochemical properties.
U.S. Pat. No. 3,525,597 describes the production of a dense polycrystalline fully stabilized cubic zirconia body characterized by a fine grain size and negligible porosity, and which is stable to oxidation at temperatures above 2000.degree. C. and is capable of transmitting visible and infrared radiation. The zirconia body consists of zirconia and about 6 mole percent of yttrium, dysprosium, or ytterbium oxide stabilizer additive. The zirconia body is prepared by hydrolytic decomposition of zirconium and yttrium alkoxides from a mutual solvent to a coprecipitate of mixed oxides, which is further processed by calcining, pressing, and sintering stages. This development of this translucent zirconia (zyttrite) is described further in Journal Of The American Ceramic Society, 50(1), 532(1967) and Ceramurgia International, 8(2), 42(1982). These publications indicate that most metal alkoxides are sensitive to moisture, heat, and light, and they vary in their hydrolytic decomposition rates, which can result in the formation of an inhomogeneous ceramic body.
U.S. Pat. No. 3,634,113 describes a zirconia composition with a cubic crystal structure that is stabilized by an additive which is a mixture of rare earth oxides
U.S. Pat. No. 4,035,191 describes a zirconia ceramic composition which is stabilized in the monoclinic and tetragonal crystal phases with 0.1-5% zinc oxide and at least about 0.5% of magnesia or yttria stabilizer additives. The zirconia ceramic composition is mechanically stable under thermal cycling conditions.
U.S. Pat. No. 4,279,655 describes magnesia partially stabilized zirconia ceramic materials. The zirconia is characterized by grains of stabilized cubic zirconia which contain discrete precipitates of tetragonal zirconia and monoclinic zirconia. A ceramic body of the zirconia exhibits high strength and good thermal shock resistance properties.
U.S. Pat. No. 4,358,516 describes .beta.-Al.sub.2 O.sub.3 type ceramic electrolytes in which fracture toughness and strength are increased by incorporating metastable grains of tetragonal zirconia in the structure. The metastable zirconia is a solid solution containing a rare earth stabilizer additive. The increased resistance to cracking is attributable to stress-induced martensitic transformation. A crack stress field causes tetragonal zirconia to transform to monoclinic zirconia, which increases the energy required for the crack to propagate and thereby increases the ceramic body resistance to fracture.
U.S. Pat. No. 4,360,598 describes zirconia ceramics stabilized with yttria, and consisting of crystal grains having a mixed phase consisting essentially of tetragonal phase and cubic phase or having a phase consisting essentially of tetragonal phase. A ceramic is produced by molding a fine powder of Y.sub.2 O.sub.3 /ZrO.sub.2 (2/98:7/93) and sintering the molded body at 1000.degree.-1550.degree. C.
A Russian publication (Materials from the 3rd All-Union Seminar, Moscow, "Science", pages 116-119, 1978) describes the production of cubic zirconia by the decomposition of a mixture of zirconium and cerium .beta.-diketonates. Thermally stable dielectric coatings and films are produced.
Thermochimica Acta, 58, 253(1982) describes the use of gelled metallorganic precursors as starting materials to produce ceria-zirconia tetragonal solid solutions. Drying the gelled precursors provides a fine powder of a quasi-amorphous nature. Upon firing, a ceria-zirconia tetragonal solid solution is obtained which sinters without the separation of individual metal oxide crystallites.
There is continuing interest in the development of zirconia ceramic materials with properties which are uniquely adapted for specialized refractory applications.
Accordingly, it is an object of this invention to provide an improved process for the production of metastable tetragonal zirconia.
It is another object of this invention to provide a fine grain metastable tetragonal zirconia powder which is free of monoclinic and cubic phase crystal structures.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.